1. Field of the Invention
This invention relates to an image fixing device for fixing a toner image formed on an image holding member such as plain paper, photosensitive paper, etc. by the electrophotographic process, electrostatic recording process, magnetophotographic process, and others.
2. Description of the Prior Art
For the image fixing device to fix a toner image on an image bearing member, there has heretofore been used mostly a device of a type which imparts heat or pressure to the toner image in direct contact with the toner image or the image bearing member to thereby fix the image.
In this type of image fixing device, e.g., a heat-fixing device, the toner image bearing member is caused to pass between a fixing roller (i.e., heating roller) and a press-contacting roller in such a manner that the toner image bearing surface of the member may contact with the heating roller. For the toner image not to offset to the surface of the heating roller, it is coated with an offset preventive material such as silicone rubber, tetrafluoroethylene resin, etc. However, when the surface temperature of the heating roller is increased for improving the image fixing capability, or when the offset preventive material on the roller surface becomes deteriorated, the toner image tends to readily offset to the heating roller, and, at its extremity, the toner image bearing member rolls around the heating roller. In order to avoid such inconveniences, it has so far been a practice to apply offset preventing liquid (liquid parting or releasing agent) onto the surface of the fixing roller by means of a felt roller, etc.
When the offset preventing liquid is coated on the fixing roller surface, it is necessary that the liquid to be applied to the surface of the heating roller or the press-contacting roller be in an appropriate quantity. For the liquid applying member, there may be used generally heat-resistant felt or heat-resistant cloth having the property of absorbing and retaining the offset preventing liquid therein utilizing capillary action. There are two methods of coating the liquid: the one is to urge the abovementioned heat-resistant felt or cloth direct by to the heating roller; and the other is to transfer the parting agent (offset preventing liquid) by urging the felt or cloth having the property of absorbing and retaining the liquid parting agent to the liquid applying roller, by which roller the parting agent is applied onto the heating roller or the press-contacting roller. However, even when the offset preventing liquid is applied to the fixing roller by the abovementioned methods, there still occurs slight offset phenomenon. For removing this offset phenomenon, it has been the general practice to provide a separate cleaning device. This cleaning device is made of heat-resistant felt, heat-resistant cloth, or the like. Even with such cleaning device, however, the cleaning effect does not reach its perfection (100%) while the heating roller is in rotation, and there remains a very small quantity of the offset toner on the fixing roller surface. The toner which stays on the heating roller by such insufficient cleaning gradually accumulates on the roller or the felt as the offset preventing liquid applying member to make it unable to uniformly coat the offset preventing liquid.